A high efficiency microreactor with Pt/ZnO nanorod arrays on the inner wall for photodegradation of phenol

Highlights

• A novel microreactor with Pt/ZnO nanorod arrays on the inner wall is fabricated.

• The microreactor shows high efficiency for photodegradation of phenol.

• The Pt/ZnO nanorod arrays display high durability during continuous recycling.

• It provides a new way to immobilize catalysts on the inner wall of a microchannel.

Abstract

A high efficiency microreactor with Pt coated ZnO (Pt/ZnO) nanorod arrays on the inner wall was successfully fabricated by pumping a Pt sol into the microchannel containing preformed ZnO nanorod arrays. Phenol was selected as a persistent organic pollutant to evaluate the photocatalytic performance of the microreactors. The microreactor which was coated by Pt sol for 5 min showed the best photocatalytic performance compared with other Pt/ZnO nanorod array-modified microreactors. The presence of Pt nanoparticles on the surfaces of ZnO nanorods promoted the separation of photoinduced electron–hole pairs and thus enhanced the photocatalytic activity. In addition, the recyclable property of the microcreator was investigated. It was found that the microreactor displayed higher durability during the continuous photocatalytic process.

Introduction

Photocatalytic degradation of organic pollutants employing ZnO semiconductors, such as ZnO nanorods [1], [2], [3], ZnO nanoparticles [4], [5], [6], ZnO nanowires [7], [8], [9], has emerged as a promising new route for degradation of persistent organic pollutants in recent years. However, several issues about ZnO photocatalysts are still unresolved when used during the photocatalytic processes.

One of the major problems for ZnO photocatalysts is the lower photocatalytic efficiency due to the quick recombination of photoinduced charge carriers [10]. In order to prevent the recombination of electron–hole pairs, rapidly transferring one of the charge carriers to a solution-phase redox couple is usually required. One probable resolving route is to combine ZnO nanocrystals with noble metal nanoparticles to form heterostructures. When noble metal nanoparticles are deposited on the surfaces of ZnO, Schottky barriers will form at the junctions and the noble metal nanoparticles will act as an electron sink which can greatly enhance the efficiency of charge separation [11], [12], [13]. Up to now, various metal-ZnO photocatalysts have been used for photocatalytic reactions and demonstrated to enhance their photocatalytic activities [14], [15], [16], [17], [18]. Besides, for metal-ZnO heterostructures, the electron transfer direction is determined by the work function of metal [19], [20]. Thus, platinum (Pt), which has a large work function of 5.93 eV, is one of the best candidates for constructing metal–ZnO heterostructure to improve the photocatalytic efficiency of ZnO photocatalyst [21], [22].

In addition, the separation and recovery of photocatalysts from the liquids is also an important problem in view of their applications in photocatalytic processes, which inevitably increases the operating cost. One way to prevent this problem is to use the photocatalyst in an immobilized form. For instance, ZnO nanorod arrays synthesized on a Si substrate by chemical vapor deposition have been used as a photocatalyst [23]. ZnO nanotube arrays electrodeposited on indium tin oxide glass substrates were used for degradation of methyl orange [24]. Although these methods can indeed solve the separation and recycle problems, their applications are limited owing to their lower photocatalytic efficiency and high cost.

The miniaturization of chemical processes using microreactors exhibits many significant advantages over existing conventional techniques, such as high surface-to-volume ratio, short diffusion distances, and rapid, efficient heat dissipation and mass transfer etc. [25], [26]. What is more, microreactors can directly be used in parallel to enable large scale production without any problems during the process of scale-up [27], [28]. Due to the advantages mentioned above, microreactors have been widely researched in many fields [29], [30], [31]. In past work, a microchannel-based reactor, which was modified with aligned TiO₂/ZnO nanorod arrays on the inner wall, was successfully fabricated by our group [32]. By fabricating this microreactor, it can easily eliminate the need to separate photocatalysts from the solutions and obviously improve the photocatalytic performance of ZnO photocatalysts at the same time. However, as mentioned before, preventing the electron–hole pair recombination is a key step for improving photocatalytic efficiency of ZnO photocatalysts. For ZnO/TiO₂ nanorod array-modified microreactor, the quick recombination of photoinduced charge carriers still exists in ZnO/TiO₂ nanorod photocatalysts and their photocatalytic performance will still be influenced. Thus, when the microreactor is going to be used for degradation of persistent organic pollutants, it is necessary to find a new way to improve its photocatalytic performance.

In this paper, a novel microreactor modified with Pt/ZnO nanorod arrays on the inner wall is constructed. The microreactor is fabricated simply by injection of Pt sol into a microchannel-based reactor which contains preformed ZnO nanorod arrays on its inner wall. Since the Pt nanoparticles on ZnO nanorods can effectively separate the photoinduced electrons and quickly transfer them to adsorbed O₂ in the aqueous solution of phenol, the microreactor shows a high efficiency for degradation of phenol.